1. Field of the Invention
The present application relates to atomic layer deposition of metal compounds that find use, for example, in lithium ion battery applications.
2. Description of the Related Art
Atomic layer deposition (ALD) of lithium-containing compounds has recently received attention due in large part to the need for conformal materials in Li ion batteries used in electronic devices. For safety reasons, all-solid-state batteries are of great interest. The power density of all-solid-state batteries can be improved by moving from planar to three-dimensional structures. Fabrication of 3D batteries is challenging because all of the materials, including electrodes, barriers and electrolyte, have to be made conformally. As ALD allows for conformal formation of materials, ALD processes for forming metal-containing compounds used in batteries, such as aluminum, titanium and lithium phosphates, as well as lithium silicates, are desirable.
Metal phosphate thin films, including titanium and aluminum phosphate films can be used in lithium ion batteries, either individually or mixed with other metal phosphates. Mixed metal phosphates can be very demanding to manufacture with precision. While ALD has been a reliable technique to deposit complex materials homogeneously over large areas and on demanding, complex structures, the development of ALD-grown phosphate thin film processes has been quite limited and only a few metal phosphate thin film processes have been reported. The common approach to incorporating phosphorous into films in these ALD processes is to use two sub-cycles, consisting of a metal oxide sub-cycle and a phosphorous oxide sub-cycle, in various ratios.
Titanium and aluminum phosphates have been shown to be applicable in lithium ion batteries either alone or mixed with other phosphates. TiP2O7 and LiTi2(PO4)3 have been examined as anode materials in lithium ion batteries. AlPO4, on the other hand, has been shown to improve the cycle-life performance of lithium ion battery cathodes. Mixed aluminum, titanium and lithium phosphates in various stoichiometries, such as Li1.3Al0.3Ti1.7(PO4)3, are promising as solid electrolytes for lithium ion batteries because of their high ionic conductivities.
Titanium and aluminum phosphate thin films or mixtures of these with other phosphates may prove to be beneficial also for some other applications. As an example, amorphous phosphates of TiP2O7 and Ce1-xTixP2O7 are promising candidates for advanced sunscreen materials in UV-shielding plastics and film applications. The broad-spectrum protection against ultraviolet radiation and chemical stability of these materials exceed those of the conventional sunscreens. TiP2O7, for example, shows only a low catalytic activity for thermal oxidation and photocatalysis.
Aluminum phosphate, notably Al(PO3)3, has been studied as a sealant of plasma sprayed lamellar chromium oxide coatings. Additionally, aluminum phosphate thin films with P/Al ratio of 1/2 have been evaluated as gate dielectrics for thin film transistors.
Lithium phosphate, Li3PO4, is known for its moderate lithium ionic conductivity. As such, Li3PO4 can be applied as an electrolyte in solid state lithium ion batteries and CO2 gas sensors. Also optical humidity sensors based on Li3PO4 have been presented recently.
In general, 3D structuring of lithium ion battery materials would be a way to effectively increase the energy storage capacity of the battery applications. As mentioned above, it would be beneficial to have atomic layer deposition (ALD) processes for depositing thin films on such demanding structures, as films grown by ALD inherently possess good conformality and uniformity because of the alternating, saturating precursor doses and self-limiting surface reactions, which lead to accurate thickness controllability and repeatability of the films. Some of the electrolyte material candidates for 3D batteries include for example Li3PO4 and nitrogen mixed lithium phosphate known as LiPON.
The use of ALD for phosphate thin film deposition has been quite scarce. However, some ALD processes for aluminum phosphate films were developed in the 1990s. Besides aluminum phosphate, ALD processes have been reported for calcium phosphate and titanium phosphate thin films. Also lithium containing ALD processes were absent until recent reports on ALD of lithium hydroxide, lithium carbonate, lithium lanthanate and lithium lanthanum titanates. In addition, ALD of LiFePO4 and Ca:LaPO4 materials have been recently presented.
However, in previous ALD metal phosphate processes, separate metal oxide and phosphorus oxide sub-cycles, both using separate oxygen sources, have been applied to incorporate phosphate into the films. This kind of approach may result in long and complicated growth cycles, which increases the total deposition time required.
Lithium silicates can be applied in lithium ion batteries as solid state electrolytes. Amorphous Li2SiO3 and Li4SiO4 films have at room temperature quite limited ionic conductivities of 2.5×10−8 Scm−1 and 4.1×10−7 Scm−1, respectively, for lithium ion battery electrolytes. However, amorphous Li2SiO3 thin films have shown to have about 1 to 2 orders higher ionic conductivity compared to polycrystalline Li2SiO3 at elevated temperatures. The ionic conductivity of amorphous Li2SiO3 can further be increased by optimizing the film thickness. Also the substrate material has been reported to affect the ionic conductivity of amorphous Li2SiO3.
Lithium silicate can form crystalline compounds with various stoichiometries. The ratios between Li2O and SiO2 vary in these stoichiometries, thus resulting in Li2SiO3 (1:1), Li2Si2O5 (1:2), Li4SiO4 (2:1) and Li8SiO6 (4:1) crystalline structures. Likewise, the properties and thus applicability of the corresponding stoichiometries vary.
Li2Si2O5 mixed with carbon black has been suggested as a cathode material for lithium ion batteries. It was found that Li2Si2O5 nanowires exhibit much higher capacity compared to 2-D or spherical 3-D nanostructures. Even more, Li2Si2O5 nanowires mixed with carbon black have a higher initial Li ion charge capacity compared to the conventional Li—Mn—O or Li—Co—O cathode material systems. Also SiO2/Li cells have been presented, where the reversible conversion reaction between SiO2 and Li2Si2O5 films results in good cycling performance and less than 0.01% per cycle capacity loss during the first 100 cycles. This kind of SiO2—Li2Si2O5 conversion system could be an attractive electrode in lithium ion batteries. Likewise, 2 nm of amorphous Li2SiO3 on top of LiCoO2 electrode has been found to enhance the rate performance of the electrode.
Lithium silicate films consisting of a mixture of Li2SiO3 and Li2Si2O5 on porous silicon may be applied for ozone detection as well due to the increase of ionic conductivity of lithium silicates in presence of ozone. The applicability of lithium silicate electrolytes for CO2 sensors has also been evaluated. It was shown that Li2Si2O5 resists 90% relative humidity at elevated temperature far better than the other lithium silicates. In addition, Li4SiO4 is unstable against CO2 forming Li2CO3 in ambient conditions while Li2SiO3 remains stable. Li4SiO4 membranes have been suggested for CO2 separation at high temperatures. Furthermore, lithium silicates, Li4SiO4 in particular, are considered as solid tritium breeding materials in fusion power technology.